Many amplifiers have transfer characteristics having a linear operating region and a non-linear operating region. Once the non-linear portion of the transfer characteristic of any amplifier is reached, any increase in input will not be matched by a proportional increase in output and gain compression occurs. Gain compression is a reduction in differential or slope gain caused by nonlinearity of the transfer function of the amplifying device. This nonlinearity may be caused by heat due to power dissipation, by overdriving the active device beyond its linear region, or other reasons.
Pre-distortion is a technique used to improve the linearity amplifiers, such as radio transmitter amplifiers. Pre-distortion inversely models the amplifier's gain and phase characteristics and, when combined with the amplifier, produces an overall system that is more linear and reduces the amplifier's distortion. In essence, inverse distortion is introduced into the input of the amplifier, thereby cancelling any non-linearity the amplifier might have.